1. Field of the Invention
The present invention relates to a data buffer for use with a fixed disk drive and more particularly to a data buffer for use with a fixed disk drive controller in a word processing system.
2. Description of the Prior Art
It is desirable in word processing equipment to have a fixed disk drive to store data and programs.
Many word processing systems currently available consist of a keyboard, a printer, a display and a floppy disk drive, all controlled by one or more microprocessors. Floppy disk drives have been shown to be cost effective means by which programs and data can be stored and accessed on demand by the operator. Compared to other forms of data storage, such as magnetic tape cassettes, magnetic cards and punched paper tape, floppy disks allow the user to access more information and to access the information faster.
It has been found, however, that for larger systems requiring more data manipulation and formatting, the floppy disk system is inadequate. Resource sharing and time sharing data or word processing systems, for example, generally require a larger data base and shorter access time than do stand alone systems. Moreover, sophisticated operations such as merging of files, sorting of data within files, and dictionary capabilities often require more memory space and shorter cycle times to be performed efficiently in real time. Finally, for complex data handling procedures, increased reliability is also required.
The relative advantages of floppy disks over magnetic tape, cards and paper tape unfortunately dissolve in the aforementioned environments. The solution to the problems inherent in floppy disk systems can be found by using one or more hard or fixed disks in conjunction with or in lieu of floppy disks.
As word processing systems become more sophisticated, the storage capacity of their mass memory mediums must increase and the speed at which data is stored and retrieved must decrease. For these reasons, one or more flexible (floppy) disks may not be adequate to access a great amount of data in a relatively short time, even if the floppy disks are dual-sided, double density.
A rigid or fixed disk, on the other hand, has a larger storage capacity and the disk drive associated with it is generally capable of rotating it at a greater velocity. It is therefore advantageous to adapt such a rigid disk for use with a word processing system. A rigid disk, and in particular a Winchester disk, is coated on both sides with a magnetic medium, so that two surfaces per disk are available for the storage of data. Each Winchester read/write head has three rails, or raised surfaces. The trailing end of the middle rail holds a magnetic core with wire coiled around for writing and reading the data. The two outer rails govern the flow of air. The force that results is sufficient to support a weight of 10 grams at a height of half a micrometer above the disk. The disks and the head assemblies in such a memory are sealed in a small chamber in which the air is continuously recirculated an filtered to exclude any dust particles larger than 0.3 micrometer in diameter.
The quantity of data that can be stored on a disk depends on how much of its surface area is magnetized for the storage of a bit. The width of the center rail of a Winchester head is approximately 20 micrometers, which corresponds to a track density on the disk of about 1,000 per inch of the radius. On a floppy disk however, the track density is only 48 tracks per inch for single density disks (96 tracks per inch for double density disks).
In high-performance memories one of the surfaces of a disk is devoted to patterns of bits that continuously yield information on the position of the head. Any deviation from the proper position causes the generation of a signal in the head that actuates a motor for repositioning. Such patterns of bits may be embedded within the stored data itself.
The number of bits that can be written along a track also is affected by limitations on both the head and the disk. As a result of all these constraints the number of reversals in magnetism along a data track in a device that records digital data by magnetic saturation and employs a Winchester head is about 10,000 per inch. The quantity of data stored ranges from about 20 million bits for one surface of a floppy disk to billions of bits for high-performance rigid disks.
The rate at which bits are written or read along a track is called the data rate. It ranges from hundreds of thousands of bits per second for floppy disk systems to 10 million bits per second for rigid disk systems. The main reason for the difference is the fact that floppy disks must rotate at lower speeds.
In order to use the speed and storage capacity of a a fixed disk drive most efficiently, it is necessary to prepare data for transfer to it. Data for transfer to the disk drive must be ready when the disk is in position to accept it. Consequently, it would advantageous to incorporate a data buffer to accept data from other parts of the word processing system and hold it temporarily until the precise instant when the data must be transferred. A simple storage register used as a data buffer is not sufficient due to the fact that at the moment the disk accepts data therefrom no new data is ready for transfer to the disk. The disk is therefore forced to wait for subsequent data to be transferred to it. Such is the disadvantage of systems using true first-in/first-out (FIFO) registers. It would be most advantageous to incorporate a data buffer which has provision, when necessary, for allowing data to be input while simultaneously allowing data to be output to the disk.
In U.S. Pat. No. 4,145,739 issued to Dunning, et al., a master/slave magnetic disk system is disclosed. A 256-byte RAM buffer is used as a FIFO register and is connected to a master disk controller and memory to buffer data to or from the disk controller or memory. Only one block of data is transferred at a time. Consequently successive blocks of data cannot be read from adjacent sectors during a single disk revolution.